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A series of 23 test holes has been drilled across Bradley, Calhoun, Ouachita, and part of Nevada Counties in southcentral Arkansas to determine the geology of the shallow water-bearing deposits of Tertiary and Quaternary age . The correlation of the lithologic and electric logs of the test holes and available electric and drillers' logs of oil-test wells indicates that: (1) the transition zone in the upper part of the Midway group may be about 85 feet thick in Nevada and Ouachita Counties; (2) the contact between the Wilcox and Claiborne groups is about 12 miles northwest of the location shown on the geologic map of Arkansas, and the Wilcox group is probably not present at the surface anywhere in Ouachita County; (3) a structural terrace extends from near the boundary between Ouachita and Calhoun Counties into western Calhoun County and (4) the Jackson group may extend in the subsurface as far west as Calhoun County and may be considerably thicker in Bradley County than has been previously supposed. The deposits of Eocene age in the report area are composed mainly of sand, silt, and clay that were deposited near the shoreline of the Mississippi trough. The presence of lignite, glauconite, and marine microfossils suggests that the formations of this age were deposited under both subaerial and submarine conditions. The deposits of Quaternary age consist of coarse terrace sand and gravel and alluvium that generally is composed of finer sand, silt, and clay.